Mary-Kate and Ashley Olsen have built a billion-dollar fashion and entertainment empire since their days as child stars — but they haven’t shared enough of their dough with the little people who grease the wheels of their business, a lawsuit claims.

The class-action suit, which alleges wage theft, says the 29-year-old celebs failed to pay about 40 past and present interns who have worked for them.

The lead plaintiff in the Manhattan Supreme Court case is former design intern Shahista Lalani, who says she was treated poorly, toiling for free doing menial tasks at the twins’ Dualstar Entertainment Group in 2012.

The Parsons School of Design grad worked in Manhattan under the head technical designer for the Olsens’ fashion label, The Row, for five months.

“She was very demanding,” Lalani recalled. “I was doing the work of three interns. I was talking to her all day, all night. Emails at nighttime for the next day, like 10 p.m. at night.”

Lalani claims she was hospitalized for dehydration because of the job’s demands.

“It was like 100 degrees outside. I’d just be sweating to death. I probably carried like 50 pounds worth of trench coats” to Row factories, she said.

“The head technical designer was like, ‘Go get my Advil. I need this and this because I’m feeling sick and I have this meeting,’ ” Lalani said.

“When we weren’t doing something, they’d be like, ‘Organize the buttons in the back by color code.’ You’re cleaning. You don’t get a set 15-minute break. You just go with their crazy flow. You just [got] caught up in the pressure,” Lalani claims.

“You’re like an employee, except you’re not getting paid. They’re kind of mean to you. Other interns have cried. I’d see a lot of kids crying doing coffee runs, photocopying stuff.”

Lalani never worked directly for the Olsens, but said she saw them occasionally at meetings and liked them.

“They’re really nice people,” she said. “They were never mean to anyone. They’re business people.”

The suit says there are many other interns in Lalani’s position, and they should have been paid the minimum wage plus overtime because they were doing the same type of jobs as the paid colleagues without receiving academic or vocational credit.

About the suit, Annett Wolf, a spokeswoman for the Olsens’ company, told The Post, “Dualstar is not aware of this,” but declined to comment on the firm’s intern policy.